Is is possible to convert a regular trombone slide into a tuning-in-slide? I know a few manufacturers currently offer a TIS version, but is it possible to convert a regular one into TIS? Thank you.
Well. You could. But why and to what effect?
The tuning-in-slide only effects the outer tubes. So cut both off a bit down on the slide below the cross bar. On the upper end, solder on a piece of tubing making the inner part of the new tuning mechanism. On the lower end, solder on a piece of tubing making the outer part. Add a mechanism to lock the tuning in place. You now have a heavier slide (some of us like the slide to be light instead). It helps if you check how others have done this, perhaps by looking at other instruments.
The purpose (to my mind at least) of having tuning-in-slide is to be able to have a different design of the bell portion of the trombone. Without a tuning slide there you avoid the same diameter portions of the tuning slide there and can let the bell flare continue further into the instrument. Some say that it makes a sonic difference, to my ears it probably is very small but sometimes a very small difference is what you are after.
The ultimate solution would probably be to make a trombone that has no tuning slide function. In my experience the tuning slide often ends up at about the same position with slight variations. As we need to tune every note anyway depending on chord function it would work with a bit of training.